Retrievable well apparatus have an outer housing and an inner mandrel telescopically received within the outer housing. In order to secure the apparatus within a well, such apparatus have packing elements, gripping slips, or both. Upon relative telescopic movement of the outer housing and the mandrel in a first direction, the packing elements or gripping slips are forced outwardly into engagement with conduit within the well. Upon relative telescopic movement in a second direction, the packing elements or gripping slips retract to enable the retrievable well apparatus to be retrieved from the well. The key component in a retrievable well apparatus is a ratchet/release mechanism that enables relative movement of the housing and the mandrel in the first direction, but prevents relative movement of the housing and the mandrel in the second direction until intentionally released.
U.S. Pat. No. 4,289,200 which issued in 1981 discloses a retrievable well apparatus developed by Baker International Corporation. The key components in the ratchet/release mechanism of the Baker apparatus include a housing, a locking ring which can be radially inwardly constricted, two diametrically opposed segments, a mandrel and a movable sleeve. The housing has an inner wall with a series of fine pitched axially spaced teeth which are configured to provide an inclined plane which permits movement in only the first direction. The teeth on the inner wall of the housing engage similar teeth on an outer cylindrical surface of a lock ring. The lock ring has an axial slot to permit radially inward constriction. The lock ring also has teeth on an inner cylindrical surface. The teeth on the inner cylindrical surface of the lock ring engage similar teeth on an outer cylindrical surface of the pair of diametrically opposed segments. The segments engage axial slots in the mandrel. The segments are radially inwardly movable in the slots by a sufficient amount to release the threads on the outer cylindrical surface of the segments from engagement with the threads on the inner cylindrical surface of the lock ring. The sleeve is axially movable along the mandrel. When the sleeve is positioned behind the segments, it holds the segments out maintaining them in engagement with the lock ring. The sleeve is retained in position by shear pins. When a retrievable well apparatus is to be retrieved, sleeve is engaged by threaded engagement with a retrieval tool. A force is then exerted to shear the shear pins enabling the sleeve to be moved axially along the mandrel. Once the sleeve is moved out of the way, the segments are able to move inwardly further into the slots and out of engagement with the teeth of the locking ring.
European Patent Specification 431,689 published in 1995 discloses a retrievable well apparatus developed by Dowell Schlumberger. The key components in the ratchet/release mechanism include a ratchet sleeve that can be collapsed radially inwardly, a split ratchet ring, a two part telescopic housing held in a set position by shear pins, and a mandrel with axial recesses. The ratchet sleeve is mounted on the mandrel. The ratchet sleeve has slotted ratchet finger elements that can be collapsed radially inwardly. The finger elements have external threads. The split ratchet ring is T shaped. The T shape enables the elements of the ratchet ring to be retained within recesses in the housing. The ratchet ring elements have cooperating internal threads that engage the external threads on the ratchet sleeve. As the mandrel moves in the first direction, it forces the split ratchet ring to expand outwardly to provide ratcheting as sleeve slides during setting. The teeth of the ratchet fingers on the ratchet sleeve are held in engagement with the teeth of the split ratchet ring due to presence of the mandrel. In order to release ratchet/release mechanism, the housing is threadedly engaged and a force exerted upon it to shear the shear pins, thereby allowing a portion of the housing to telescope. The telescoping portion of the housing has an inwardly projecting portion which cams against the ratchet fingers of the ratchet sleeve. Mandrel has a reduced outer diameter portion into which the ratchet fingers are pushed by this camming action. Once the ratchet fingers have been pushed sufficiently inwardly they disengage from the trapping threads on the split ratchet ring.
The Baker ratchet/release mechanism and the Dowell Schlumberger ratchet/release mechanism described above were developed for positioning between production tubing of a well and well casing. In this environment the smallest outer diameter that one would work with would be 41/2 inches. In this environment the normal manner of connection is a threaded connection. In this environment the force required to trigger the release of the ratchet/release mechanism is typically in the range of 30,000 to 50,000 pounds.
Their are applications in the oil and gas industry in which it would be advantageous to have a retrievable well apparatus that could be retrieved by slick line. Such application generally involve setting tools within the production tubing. The ratchet/release mechanisms described above cannot be used for such applications. There are practical limits as to the extent to which the described ratchet/release mechanisms can be miniaturized. There are also limits that are inherent in the use of a slick line. The major limitation relating to the force required to release the ratchet/release mechanism. A slick line is only capable of safely applying a release force in the magnitude of 1000 to 2000 pounds.